[spec] adding a "magic number" for gemini files

[Sean Conner]

It was thus said that the Great cage once stated:
> On Sat, Dec 26, 2020 at 06:25:43PM -0500, Sean Conner wrote:
> 
> [...]
> 
> > > I was  thinking abo[u]t file(1) and  libmagic in general.
> >
> >   I use libmagic for my server [1] and yet, I still use an extension to mark
> > a Gemini text file.
> 
> Do you mean you check the file extension for recognize a gemini file?
> Or  you use  same kind  of heuristic  to prevent  libmagic to  returns
> gemini file as text?

  In the configuration file, the user can specify a mapping of extensions to
MIME type, to which the default extension type that identifies a Gemini
index file is added (which for my server is ".gemini" but that can be
changed).

  During a request, the extension is checked against this mapping.  If (for
a non-Gemini file) this doesn't exist, only then is libmagic queried.

  -spc